1. Field of the Invention
The present invention relates to a display apparatus in which a display device and an integrated circuit for driving the display device are mounted on the same substrate, wherein the substrate is connected to an external circuit by a flexible printed circuit.
2. Prior Art
Nowadays, many portable apparatuses, such as portable telephones, are equipped with a display apparatus comprising a display device, such as a liquid crystal display device, and an integrated circuit for driving the display device. In such a display apparatus, however, as the number of terminals on the display device increases, it becomes difficult to drive the display device by a single integrated circuit, and therefore, it is practiced to drive the display device by using a plurality of integrated circuits, as disclosed in prior art such as Japanese Unexamined Patent Publication No. 11-338438.
When driving a display apparatus by using a plurality of integrated circuits as described above, it is preferable, from the standpoint of design, to use integrated circuits having output terminals the number of which is chosen to match the number of terminals on the display device, that is, custom integrated circuits. This, however, leads to the problem that the cost increases because the custom integrated circuits must be designed and fabricated to match the number of terminals on the display device to be used. It is therefore practiced, as in the above prior art, to reduce the cost by employing general-purpose integrated circuits each having a predetermined number of output terminals and by choosing the number of such integrated circuits so that the total number of their output terminals exceeds the number of output terminals on the display device. In this case, as the total number of output terminals on the plurality of integrated circuits naturally becomes larger than the number of terminals on the display device, various methods are used to handle the terminals that are not brought out and used for driving the display device.
On the other hand, to meet the needs for smaller and lighter portable apparatuses in which a display apparatus such as described above is to be mounted, process improvements have been made in general-purpose integrated circuits, and high-performance and low-cost integrated circuits have been achieved by increasing the number of outputs while achieving a reduction in overall size. As a result, while conventional general-purpose integrated circuits have a structure such that the output terminals are formed only on one longer side (hereinafter called the first side) of the rectangular chip and the input terminals on the other longer side (hereinafter called second side) opposite the first side, small and multi-output integrated circuits have been commercially implemented that have output terminals not only on the first side but also on the second side on which the input terminals are formed, thereby increasing the total number of output terminals.
Referring to drawings, a description will be given below of how the integrated circuit terminals not brought out for connection are handled in a display apparatus constructed using general-purpose integrated circuits that have output terminals on the second side also. The following description is given for the case of a liquid crystal display apparatus which uses a liquid crystal display device as the display device.
FIG. 5 is a diagram showing the wiring structure of the liquid crystal display apparatus that employs a packaging technique known as tape-automated bonding (TAB) in which small, multi-output general-purpose integrated circuits, such as those described above, are mounted on flexible printed circuits (hereinafter called the FPCs) provided separately from the common substrate. First, the construction of the liquid crystal display apparatus will be described. The liquid crystal display apparatus comprises a printed circuit board 422 (hereinafter called the PCB) on which a signal generating circuit and power supply circuit (neither shown here) for driving the liquid crystal display are mounted, a first FPC 420 on which a first integrated circuit 409 is mounted, a second FPC 421 on which a second integrated circuit 410 is mounted, a third FPC 419, and a liquid crystal display device 435 constructed by injecting a liquid crystal between a device substrate (hereinafter called the first substrate) 418 and the common substrate (hereinafter called the second substrate) 434. Thin-film transistor (TFT) devices are formed on the first substrate 418, and a whole-area electrode is formed in a display area 430 on the second substrate 434. Many commercially implemented liquid crystal display apparatuses have high-resolution display capabilities (for example, 640 columns by 240 rows) but, for simplicity of explanation,-the following description assumes that the display apparatus has a matrix structure of 70 columns and 20 rows.
The first FPC 420, second FPC 421, and third FPC 419 as connecting boards are connected to the PCB 422 as an external circuit by crimp contacts. The first FPC 420, second FPC 421, and third FPC 419 are attached to the first substrate 418 by thermal compression using an anisotropic conductive sheet (ACS) formed by mixing conductive particles in an insulating adhesive sheet.
The first FPC 420 has the function of transferring signals generated by the signal generating circuit and power generated by the power supply circuit from the PCB 422 to the first integrated circuit 409 and transferring outputs of the first integrated circuit 409 to the first substrate 418. The second FPC 421 has the function of transferring signals generated by the signal generating circuit and power generated by the power supply circuit from the PCB 422 to the second integrated circuit 410 and transferring outputs of the second integrated circuit 410 to the first substrate 418. The third FPC 419 has the function of transferring signals generated by the signal generating circuit and power generated by the power supply circuit from the PCB 422 to the third integrated circuit 417 and transferring outputs of the third integrated circuit 417 to the first substrate 418.
The liquid crystal display device 435 is supplied with data signals from the first integrated circuit 409 and second integrated circuit 410 and a scanning signal from the third integrated circuit 417, and displays an image in the display area 730 by driving the matrix of 70 columns and 20 rows in time-division line sequential fashion (multiplex driving).
As earlier described, the first integrated circuit 409 and the second integrated circuit 410 are general-purpose integrated circuits that have output terminals on the side thereof on which the input terminals are also provided. These integrated circuits 409 and 410, with their gold-plated or soldered electrode side facing down, are attached to the first FPC 420 and the second FPC 421, respectively, by thermal compression using an anisotropic conductive sheet (ACS) (this mounting method is called tape-automated bonding (TAB)). In actual commercialized versions of the first integrated circuit 409 and the second integrated circuit 410, the number of output terminals formed on the side (second side) on which the input terminals are provided is 20 or more on each of the right and left sides of the input terminals, but for simplicity of explanation, the following description assumes that there are four output terminals to the left of the input terminals and 12 to the right.
The wiring will be described in detail below. A first power supply line group 401 is used to supply power, ground (0 V potential) and +5 V potential, for driving the first integrated circuit 409 and the second integrated circuit 410. A first data signal line group 400, which is used for transferring a signal group representing grayscale, consists of four data lines, that is, the zeroth bit data line, the first bit data line, the second bit data line, and the third bit data line. A clock signal line 403 is used for transferring a signal that defines the timing for reading the signals transferred via the first data signal line group 400.
A start signal line 423 is used for transferring a start signal that defines the timing for starting the reading into the first integrated circuit 409 of the data signal group transferred via the first data signal line group 400. A first cascade signal line 405 is used for transferring a cascade signal, which occurs when the data read to the first integrated circuit 409 is completed, to the second integrated circuit 410 as a signal that defines the timing for starting the reading of the data signal group transferred via the first data signal line group 400. A latch signal line 402 is used for transferring a latch signal that defines the timing for causing the data loaded into the first integrated circuit 409 and the second integrated circuit 410 to be output. A second cascade signal line 425 is provided to transfer a cascade signal, which occurs when the data read to the second integrated circuit 410 is completed, to the next integrated circuit as a signal that defines the timing for starting the reading of the data signal group transferred via the first data signal line group 400, but, in the liquid crystal display apparatus shown in FIG. 5, as there is no further integrated circuit, the second cascade signal line 425 is not connected to any electrode on the PCB 422. (In FIG. 5, the mark x attached to the second cascade signal line 425 indicates that the line is not connected to any electrode on the PCB 422.)
A first output line group 431 indicates a plurality of output wiring lines formed along the side (second side) on which the input terminals of the first integrated circuit 409 are provided, and output signals for the first to fourth columns, as viewed from the side on which the third integrated circuit 417 is mounted, are transferred via these output wiring lines to the corresponding electrodes (not shown) formed on the first substrate 418. A second output line group 411 indicates a plurality of output wiring lines formed along the side (first side) opposite the side on which the input terminals of the first integrated circuit 409 are provided, and output signals for the fifth to 32nd columns, as viewed from the side on which the third integrated circuit 417 is mounted, are transferred via these output wiring lines to the corresponding electrodes (not shown) formed on the first substrate 418. A third output line group 412 indicates a plurality of output wiring lines formed along the side (second side) on which the input terminals of the first integrated circuit 409 are provided, and output signals for the 33rd to 44th columns, as viewed from the side on which the third integrated circuit 417 is mounted, are transferred via these output wiring lines to the corresponding electrodes (not shown) formed on the first substrate 418.
A fourth output line group 413 indicates a plurality of output wiring lines formed along the side (second side) on which the input terminals of the second integrated circuit 410 are provided, and output signals for the 45th to 48th columns, as viewed from the side on which the third integrated circuit 417 is mounted, are transferred via these output wiring lines to the corresponding electrodes (not shown) formed on the first substrate 418. A fifth output line group 414 indicates a plurality of output wiring lines formed along the side (first side) opposite the side on which the input terminals of the second integrated circuit 410 are provided, and output signals for the 49th to 70th columns, as viewed from the side on which the third integrated circuit 417 is mounted, are transferred via these output wiring lines to the corresponding electrodes (not shown) formed on the first substrate 418.
A sixth output line group 432 indicates a plurality of output wiring lines formed along the side (first side) opposite the side on which the input terminals of the second integrated circuit 410 are provided; these output wiring lines are originally provided for transferring output signals for the 71st to 76th columns as viewed from the side on which the third integrated circuit 417 is mounted, but actually these output lines are not connected to any electrodes on the first substrate 418, as the total number of columns is 70 according to the specification of the liquid crystal display apparatus shown here. (In FIG. 5, the mark x attached to the sixth output line group 432 indicates that these output lines are not connected to any electrodes on the first substrate 418.) A seventh output line group 408 indicates a plurality of output wiring lines formed along the side (second side) on which the input terminals of the second integrated circuit 410 are provided; these output wiring lines are provided for transferring output signals for the 77th to 88th columns as viewed from the side on which the third integrated circuit 417 is mounted but, actually, these output lines, like those in the output line group 432, are not connected to any electrodes on the first substrate 418, because the total number of columns is 70 according to the specification of the liquid crystal display apparatus shown here. (In FIG. 5, the mark x attached to the seventh output line group 408 indicates that these output lines are not connected to any electrodes on the first substrate 418.)
The third integrated circuit 417 is mounted on the first substrate 418 by using the so-called chip-on-glass technique, that is, by thermal compression using an ACF. The third integrated circuit 417 has the function of sequentially outputting scanning signals in response to signals input via the third FPC 419. An actual commercialized version of the third integrated circuit 417 is usually provided with 120 or more output terminals, but for simplicity of explanation, the following description assumes that it is provided with 20 output terminals.
A second power supply line group 406 is used to supply power of ground (0 V potential), +5 V, −15 V, and +15 V for driving the third integrated circuit 417. A second data signal line group 407 is used for transferring a frame start signal and a row clock signal to be input to the third integrated circuit 417. The row clock signal is a signal for defining the timing of row selection, while the frame start signal indicates the timing for selecting the first row. Scanning electrodes 416 are 20 electrodes formed on the first substrate 418 to sequentially select the rows in the liquid crystal display device 435. When the frame start signal is input, the third integrated circuit 417 sequentially selects the scanning electrodes 416 from the top to the bottom at the rising edge of the clock signal. A base signal line 436 is used for transferring to the second substrate 434 the power that defines the potential of the whole-area electrode of the second substrate 434 necessary for TFT operation. The meanings of the power supply potential and base potential for the third integrated circuit 417 are the same as those used for ordinary TFT operation, and will not be described here since they have little relevance to the present invention.
Next, the operation of the first integrated circuit 409 and the second integrated circuit 410 will be described. When the signal from the start signal line 423 is input, the first integrated circuit 409 reads the data signals on the first data signal line group 400 in synchronism with the signal rise timing of the clock line 403. When the reading of the data signals (data for the first to 44th columns) for 44 outputs, the maximum number of outputs, is completed, a cascade signal is output on the first cascade signal line 405. When the cascade signal from the first cascade signal line 405 is input, the second integrated circuit 410 reads the data signals on the first data signal line group 400 (data for the 45th to 70th columns) in synchronism with the signal rise timing of the clock line 403.
In this way, an image formed from 70 columns and 20 rows can be displayed in the display area of the liquid crystal display device. The configuration shown in FIG. 5 above is the same as the configuration disclosed in FIG. 37as a prior known example in the earlier cited prior art Japanese Unexamined Patent Publication No. 11-338438; here, all the terminals on the integrated circuit 409 are used, and the terminals not used (unconnected terminals) are all assigned to the integrated circuit 410. As a result, extra space is needed on the liquid crystal display apparatus for those unused terminals. When the invention disclosed in Japanese Unexamined Patent Publication No. 11-338438 is applied to the liquid crystal display apparatus shown in FIG. 5, a general-purpose integrated circuit is used as the integrated circuit 409 and all the output terminals thereof are used, while for the integrated circuit 410, a smaller-size integrated circuit having fewer output terminals is custom made, eliminating the need for an extra space on the liquid crystal display apparatus.
Next, the problem with the display apparatus of the prior known configuration shown in FIG. 5, in particular, the wiring structure for each integrated circuit, will be described in further detail with reference to FIGS. 6 and 7. The display apparatus shown in FIGS. 6 and 7 is a modified example of the display apparatus shown in FIG. 5; in the illustrated example, the display device and the integrated circuits for driving the display device are mounted on the first substrate by using a COG technique, and the first substrate is connected to an external circuit by a flexible printed circuit. FIG. 6 is a plan view of an essential portion showing a layout of electrode patterns on the first substrate, and FIG. 7 is an enlarged plan view showing the pattern layout for one integrated circuit.
In FIG. 6, of the two glass substrates forming the liquid crystal display device, the bottom substrate forming the first substrate 418′ is a large-size substrate on which the two integrated circuits 409 and 410 and mounted and the electrode patterns formed; on the other hand, the top substrate forming the second substrate 435 is smaller in size and defines the display area 430. In the space between the display area 430 and one end side 204 of the first substrate 418′ are mounted the small-size, multi-output general-purpose integrated circuits 409 and 410, each rectangular in shape, with their first sides 409a and 410a facing the display area 430 and second sides 409b and 410b facing the one end side 204.
On the first substrate 418′ are patterned: an output electrode group Do1 connected to a first output terminal group To1 formed on the first side 409a, 410a of the integrated circuit 409, 410; output electrode groups Do2 and Do3 connected to a second output terminal group To2 and a third output terminal group To3, respectively, formed near the respective ends of the second side 409b, 410b; and an input electrode group Di connected to an input terminal group Ti formed on the center portion of the second side.
Further, as shown in FIG. 7, routing electrode portions Dh2 and Dh3 are patterned via which the output electrode groups Do2 and Do3 connected to the second output terminal group To2 and third output terminal group To3 formed on the second side 409b of the integrated circuit 409 are routed to the same side as the first side; on the other hand, the input electrode group Di has a narrow portion Dis, which is connected to the input terminal group Ti, and a wide portion Diw, which is connected to connecting electrodes on the flexible printed circuit.
FIG. 8 is an enlarged plan view of the input electrode group Di shown in FIG. 7; as shown, in the narrow portion Dis, each pattern width 203 is 10 μm and the gap 203g is 10 μm, while the first to ninth input electrodes 221 to 229 in the wide portion Diw require a relatively large pattern width and gap in order to provide reliable electrical connections between the input electrode group Di and the respective electrodes (not shown) on the FPC 131 while preventing leakage between the patterns. In the illustrated example, about 300 μm must be provided for the pattern width 201 and 100 μm for the gap 202.
As described above, when designing a small-size display apparatus by using small-size, multi-output integrated circuits each having output terminals also on the second side on which the input terminals are provided, and by mounting these integrated circuits on the common substrate of a liquid crystal display device or the like, it is generally practiced to assign the unused, and hence unconnected, terminal group Tnc to one particular integrated circuit (410), while using all the terminals on the other integrated circuit (409), as shown in FIGS. 5 to 7. This arrangement, however, requires an extra space on the common substrate to accommodate the wide portions of the input terminals of the integrated circuit, and the provision of such space makes it difficult to reduce the size of the display apparatus.
That is, as the electrode patterns on the second side 409b of the integrated circuit 409 are formed along the entire length of the second side as shown in FIG. 7, the space for forming the wide portion Diw of the input electrode group Di cannot be secured between the second and third output terminal groups To2 and To3. As a result, the wide portion Diw of the input electrode group Di must be formed outside a formation range L2′ in which the routing electrodes Dh2 and Dh3 are formed, and hence, the distance Li′ from the second side 409b of the integrated circuit 409 to the one end side 204 of the first substrate 418′ increases, making it difficult to reduce the size of the display apparatus. Furthermore, as electrodes are brought out for all the terminals in the third output terminal group To3 of the integrated circuit 409a, the length L2′ necessary for routing these electrodes to the display device increases in proportion to the number of output terminals.